In hemodialysis it is common to dialyse a patient three times per week, with time periods of three to four hours per treatment. The object of this treatment is to provide an adequate dose of dialysis to the patient. Such a dose of treatment can be defined in different ways.
One commonly used definition uses the urea molecule as a marker molecule and prescribes that the dialysis clearance (K) divided by the distribution volume (V) of urea times total treatment time (t) should exceed a certain constant, for example Kt/V is greater than one per treatment. The weekly dialysis dose is then Kt/V greater than three.
One common way of measuring Kt/V is by measuring the concentration of urea (c.sub.b) in the plasma before and after the treatment. The ratio R=c.sub.bpost /c.sub.bpre is correlated to Kt/V. A number of different equations have been suggested for the calculation of Kt/V, such as: EQU Kt/V=-ln (R-0.03)+(4-3.5.multidot.R).multidot.UF/W (1)
where UF=ultrafiltration volume removed in liters and W=post-dialysis weight in kg.
Several clinical studies have been performed evaluating Kt/V in which post-dialysis plasma urea (c.sub.bpost) has been measured immediately after the dialysis, usually less than two minutes after ending the treatment. However, most patients have a rebound of c.sub.bpost. If an equilibrated post-treatment (c.sub.bpost) is measured after about 30 minutes, for example, a more "true" Kt/V can be measured.
The measurement of c.sub.b is not unproblematic. It is required that a blood sample be taken before and after the dialysis treatment. Such samples are then analysed by the hospital's laboratory. The resulting values are thus provided with a substantial time delay. In this way it is not possible to adjust the actual treatment so that a prescribed dose is obtained.
The post-treatment sample must be taken with care, especially regarding timing, to avoid false values due to cardiopulmonary or access recirculation. Another source of error is the rebound mentioned above.
If an equilibrated post-treatment sample should be taken, such sample should be taken about 30 to 60 minutes after terminating the treatment, which is not practical for the patient. The amount of rebound and the rate of rebound varies considerably from patient to patient.
These problems have been addressed in the prior art in different manners.
International Application No. WO 94/08641 describes the use of a urea monitor for assessing the adequacy of an dialysis treatment. The urea monitor is connected to the dialysis effluent line and measures the concentration of urea in the dialysate leaving the dialyzer.
According to this specification, it is necessary to know or measure the pre-dialysis plasma urea value (c.sub.bpre). Such measurements can be made by measuring the urea concentration in an equilibrated sample taken before initiation of the treatment. However, such initial measurement takes time and the dialysis machine needs to be specially constructed to obtain such pre-dialysis urea value.
Other indicators of adequate dialysis are URR and SRI: EQU URR=1-R=1-c.sub.b post /c.sub.b pre (2) EQU SRI=(m.sub.urea pre -m.sub.urea post)/m.sub.urea pre (3)
where m.sub.urea pre and m.sub.urea post are pre and post amounts of urea in the body, respectively.
The object of the present invention is to provide a method and apparatus for determining the efficiency of a dialysis treatment and monitoring delivered dose of treatment on-line.
Another object of the present invention is to provide a method and apparatus for continuously monitoring the dialysis efficiency for adjusting the dialysis treatment on line when required, for example if the dialyzer is clotted.
A further object of the present invention is to provide a method and apparatus for estimating the dose of dialysis delivered without the need for taking blood samples or requiring the dialysis machine to make any special adjustments such as taking an equilibrated pre-dialysis plasma urea concentration.